1. Field of the Invention
The present invention relates to an electrical interconnection system of the type using a "mother" card to interconnect a plurality of "daughter" cards.
2. Description of Related Art
In equipment or installations that include a very large number of electrical or electronic components, the components are generally mounted on cards, i.e. on rigid insulating supports that include conductive tracks for interconnecting the various components mounted on the same card. The card may be single sided or double sided. The equipment or installation naturally includes a plurality of cards.
To interconnect the various cards, i.e. to interconnect the various circuits implemented on the various cards, it is general practice to use a mother card. The mother card is installed and fixed horizontally in a mechanical support referred to as a "chassis". To provide electrical connection between each of the so-called daughter cards and the mother card, the mother card includes mutually parallel bases, and each daughter card includes a connector, with each connector having electrical connection elements that co-operable with corresponding elements in the associated base when the daughter card is installed on the mother card. Co-operation between the base and the connector also serves to provide one of the mechanical connections between a daughter card and the mother card.
In order to enable the various daughter cards mounted on a common chassis to be interconnected to make up a desired circuit, it will be understood that the mother card must itself include conductive tracks or conductor elements serving to electrically interconnect the various conductor elements of the various base connectors appropriately and also to connect them to output terminals of the mother card, i.e. output terminals of the chassis as a whole, i.e. of the circuit constituted by the mother card and the daughter cards connected thereon.
FIGS. 1 and 2 show one such known interconnection system. FIG. 2 is a perspective view of a portion of a mother card 10 with two bases 12 and 14 secured thereto. In this figure, there can be seen a row of connection elements given respective references 16 and 18. There can also be seen two daughter cards 20 and 22 having respective connectors 24 and 26. The connectors are provided with their own connection elements 28 and 30 which co-operate with the connection elements 16 and 18 of the bases 12 and 14 when the daughter cards 20 and 22 are installed on the mother card 10.
FIG. 1 is a diagram of the electrical connections that can be implemented in this way. There can be seen electrical conductors 32 and 34 which connect the connector elements 28 and 30 of the connectors 24 and 26 to conductor tracks (not shown in this figure) of the daughter cards 20 and 22.
There can also be seen the conductors 36 and 38 which connect the connection elements 16 and 18 of the bases to bus-forming conductors 11 of the mother card 10.
In particular, it can be seen that all of the electrical interconnections implemented by the mother card 10 transit via the connection elements between the connectors and the bases.
The mother card which supports the bases that in turn receive the connection elements of the connectors associated with the daughter cards constitutes one of the limits on increasing the level of integration provided by the chassis of which it forms a part. At present, interconnection density in an electronic chassis is increased by adding contacts to the bases and the associated connectors, by miniaturizing the contacts, by adding plated-through holes (PTHs), and by adding electrical conductor layers within the mother card using finer and finer etching line thicknesses to define the conductive tracks. The mother card thus becomes less and less reliable both mechanically and electrically, and in addition more and more expensive because of its greater complexity.
When it is desired to increase the component density of the chassis, various mechanical limitations related to implementing the mother card are encountered. Of such limitations, particular mention may be made of the following:
internal track width; PA1 density of internal conductor tracks; PA1 hole diameter prior to metal plating; PA1 density of plated holes in the mother card; PA1 number of conductor layers in the mother card; PA1 the material from which the card is made; PA1 the planeness of the card; and finally PA1 the dimensions of the mother card.
Increasing component density also encounters a certain number of limits concerning the electrical characteristics of the mother card. Travel time via a mother card is no longer negligible when the length of the conductor tracks need to be increased. It is therefore advantageous to attempt to achieve connections that are as short as possible. In addition, the characteristic impedance of a line makes it possible to ensure that the shape of the signal travelling via a conductor is conserved. However applying impedance constraints would merely add further constraints on the design of circuits in a mother card. Furthermore, increasing track density gives rise to increased probability of cross-talk. Finally, increasing the number of plated-through holes increases the number of tabs associated therewith and gives rise to non-negligible capacitance.
It can thus be seen that there exists a real need to provide an improved way of interconnecting daughter cards by means of a mother card for two purposes: either for given component density to reduce the density of connections on the mother card so as to improve the above-outlined characteristics thereof; or else while maintaining the same quality of characteristics as mentioned above, to further increase the connection density of circuits on the daughter cards.